Cellulosic product



July 24, 1962 s. c. ROGERS 3,046,171

CELLULOSIC PRODUCT Filed Oct. 27, 1958 Uite 3,046,171 Patented July 24, 1962 3,946,171 CELLULGSIG PRODUQT Sedgwick C. Rogers, Appleton, Wis, assignor to Kimberly-Clark Corporation, Neenah, Wis., a corporation of Delaware Filed Get. 27, 1958, Ser. No. 769,781 6 Claims. (Cl. 154-453) The present invention relates generally to the manufacture of particle board and similar lignocellulosic sheet materials and has for its principal object the improvement of the surface characteristics of these materials.

As will hereinafter appear, this object is accomplished by the provision of a composite masking or surfacing laminate of novel type which is adapted to be attached to the sheet material with which it is used so as to constitute the exposed surface thereof.

Very large amounts of lignocellulosic sheet material or boards are manufactured and used each year in the United States for buildings, furniture, and other similar uses. While a large amount of such boards, particularly the more expensive types, are made from large pieces of wood or veneers and are generically referred to as plywood, there is also a substantial proportion of lignocellulosic sheet or board which is made by pressing lignocellulosic fibrous material or comminuted bits of material into a composite sheet or board. This latter material may be described by the generic term particle board.

In the manufacture of particle board and related products, lignocellulosic materials such as wood or agricultural residues such as bagasse, corn stocks, straw and the like are first reduced to fiber clumps or to comminuted pieces. These are then formed into a predetermined shape, usually sheet form which is then consolidated by the application of heat and pressure. While the lignocellulosic materials may be consolidated merely by the application of heat and pressure employing the natural binding material found in the raw materials, a certain amount of resinous binder such as phenol aldehyde thermosetting resin is usually incorporated with the lignocellulosic material to aid in the consolidation thereof.

The resultant lignocellulosic board product has certain advantages for particular uses over other forms of composite board products. It is usually characterized by good strength and dimensional stability and because of the use of comminuted particles it is highly resistant to checking and curling. It is also usually quite inexpensive because the raw materials from which it is made are often materials which would otherwise be wasted or in the case of Wood could not be used in plywood because of structural or appearance imperfections. These materials often, however, are unsuitable for many uses because of the appearance of the board surfaces. Because of the nature of the particles from which the board is made the surface of the boards does not present a pleasing appearance in general. This is particularly true if boards are made from agricultural residue or from wood bits which are normally waste material such as sawdust, Wood flour, ground wood, shavings, chips, etc.

Various methods have been employed to improve the surface characteristics of such particle board but these have in general not been widely adopted, usually because the expense of the surfacing method defeated the purpose of obtaining a relatively inexpensive board material. For example, one of the most commonly used methods of improving the surface is by painting it. However, in general, particle boards are not amenable to this type of surfacing treatment because the boards do not have a smooth uniform surface. Each chip or particle forming the surface of the board may have a different penetration rate, drying rate, and other surface characteristics such as hardness which Will result in diflferences in reflection and in general the appearance of the painted surface will be poor and uneven.

Another method of surfacing particle board is to apply resin to the surface. In general, however, this method is expensive if the resin is applied to the board in suflicient quantity to provide a smooth surface. There is also the problem of masking the appearance of the board by the resin. If the resin is made opaque this further increases the expense of the resin coating. Furthermore the resin coatings in general do not provide a surface which accepts paint readily so that the resin coated particle boards are not amenable to masking by painting. There are resin impre nated masking sheets on the market which may be used to cover particle board and provides a smooth surface. These sheets, however, if used in a basis weight sufficiently heavy to provide a smooth surface and to mask the surface of the particle board are quite expensive and furthermore since the surface is primarily plastic are not readily amenable to painting. Another Way of upgrading particle board is to use it as a core for fine veneers, but this also results in a comparatively expensive product.

In the practice of the present invention a surface layer is applied to the particle board. This surface layer is comprised of a greaseproof paper surfacing sheet and a resin impregnated paper attachment sheet said sheets having certain very definite characteristics as will hereinafter appear and when applied to said particle board the surfacing layer effectively masks the surface of the board and provides a smooth, paintable surface therefor. The principal object of the invention is thus accomplished. Furthermore the principal object is accomplished in a most inexpensive manner. Other objects of the present invention will be apparent from the following description thereof.

FIGURE 1 is a perspective view illustrating a completed lignocellulosic board panel which has ben provided with a smooth, paintable surface by the use of a composite laminate material in accordance with the present invention.

FIGURE 2 is a fragmentary, enlarged, perspective view of a portion of the panel illustrated in FIGURE 1.

In the surfacing of particle board panels in accordance with the process of the present invention, the finished article is manufactured by placing the resin impregnated attachment sheet on the surface of a particle board panel. The greaseproof paper sheet is then placed on the resin impregnated paper attachment sheet and the combination of particle board panel, resin impregnated attachment sheet and greaseproof paper surfacing sheet is placed in a conventional plywood-type hot press where the combination can be subjected to heat and pressure. It is of course apparent that one or both surfaces of the particle board panel may be surfaced at the same time.

The drawings illustrate the completed product made in accordance with the present invention.

FIGURE 1 illustrates a typical surfaced panel of particle board 10 which has been surfaced with a composite laminate of a resin-impregnated paper attachment sheet and a greaseproof surfacing sheet 13. An enlarged sectional view of the panel is illustrated in FIGURE 2 wherein the surfaced panel ltl is comprised of a particle board layer it, a resin impregnated attachment sheet 12 and a greaseproof paper surfacing sheet 13.

The resin impregnated attachment sheet of the present invention serves at least two purposes. First, it serves to attach the greaseproof surfacing sheet uniformly to the surface of the particle board panel and secondly it serves to provide a smoother surface for the panel. This is accomplished because the resin impregnated sheet during the heating and compression of the attachment step will tend to fill any interstices in the surface of the board thus providing a cushioning and smoothing effect and resulting 3 in a very smooth outer surface of the attached sheet. The resin impregnated attachment sheet will also provide a certain amount of masking, partially because of its opacity and also because it tends to smooth out any imperfections of the surface of the board.

These results are accomplished by the use of various types of base sheets and by the use of various types of impregnating resins. For example, a base paper or the resin impregnated sheets may vary according to the pulp used in the furnish and the method of manufacturing. The base sheet may be made with kraft, sulfite Mitscherlich or soda chemical pulps and mixtures of these pulps. Other chemical pulps may also be used. Groundwood pulp or semi-chemical pulp may be incorporated into a mixture with the chemical pulp in order to improve the formation, cushioning effect, etc. Cold caustic pulps may also be incorporated. Generally speaking, the base sheet which is to be impregnated should be capable of absorbing the resin impregnant in suitable amounts and should be sufiiciently bulky and resilient so that it will provide an adequately cushioned substrate for the surfacing sheet. For most surfacing applications, base sheets having a basis weight of from about 25 to 100 pounds per 3000 square feet and containing a substantial percentage of chemical pulp will be found most satisfactory. Such sheets having a caliper from about 3 to 10 mils, and preferably 5 to 8 mils, and a density in the range of from about 0.5 to about 0.8 gram per cubic centimeter can be readily impregnated by passage through a bath containing the impregnating resin to provide the desired resin content. Paper having a basis weight of from about 35 to 60 pounds per 3000 square feet is preferred for most applications. It will, of course, be understood that while a single resin impregnated sheet is preferred for the attachment sheet, the attachment layer may be made up of a plurality of such sheets. Now while this is generally less convenient and a more expensive way to manufacture the finished product there may be special applications where the use of a plurality of sheets is desirable. In such cases the various characteristics such as basis weight, etc., which are described refer to the total weight of the sheets employed in the layer rather than to the individual sheets. A plurality of sheets, which may be of somewhat different types is usually preferable to a single very heavy sheet.

The resin employed to impregnate the attachment sheet should be selected from the general class of resins known as laminating resins. Also the resin should be of the thermos'etting type, which is to say that the resin should be one in which the curing reaction is irreversible: in character and once the resin has hardened will not return to its original fluid state upon heating. The most commonly used laminating resins for this purpose are the aldehyde resins such as the phenol aldehyde and resorcinol aldehyde resins. Water-soluble phenolic resins have been found to be particularly suitable. These include resins derived from mono and polyhydric phenols and their derivatives (cresols and cresylic acids and its derivatives) reacted with aldehydes and aldehyde yielding compounds, for example, formaldehyde (including formalin, paraformaldehyde, and hexamethylenetetramine) furfural, etc.

Generally it is desirable that the impregnating resin shall be a relatively low viscosity, high solids content resin. Resins having a viscosity within the range of from about 100 to 250 centipoises at a temperature of 75 F., and solid contents within the range from about 6668 percent by weight 'are suitable. These resins usually have a specific gravity within the range of about 1.10 to 1.30 and are normally supplied at a pH range of about 7.0-8.5.

Where the resin impregnant of the attachment sheet is relied upon to establish the bond between the surfacing sheet, the attachment sheet, and the particle board the resin employed should be one which will not become fully set during the impre nation and drying of the sheet or for a reasonable shelf life period thereafter prior to the attachment operation. The ability of the resin impregnated attachment sheet to act as a bonding agent under normal attachment conditions may be expressed in terms of its flow characteristics. In a common test for this characteristic a stack of sheets of resin impreghated paper, each sheet 4 inches square, and the stack weighing a total of 32 grams, is heated for 7 minutes in a press at 150 pounds per square inch at a temperature of 145 C. The resin which has flowed from the impregnated paper sheets is then removed and the consolidated stack of sheets reweighed. The percentage loss in weight during this test is referred to as the percentage of flow. Thus a sample of impregnated paper tested by this method, which loses 5 percent in weight is spoken of as having a 5 percent flow. For the purpose of the present invention the resin impregnated attachment sheet should retain a flow of at least about 2 percent during the impregnation, drying period and for a reasonable storage period thereafter, for example from 6 to 12 months. The setting characteristics of most phenol formaldehyde resins are suitable without modification. With other aldehyde laminating resins, however, it may be desirable to employ suitable polymerization inhibitors such as are known to the resin art.

The optimum resin contents when using base sheets of the type described above (i.e. moderately absorbent sheets having a basis weight within the range of from about 25 to 100 pounds per 3000 square feet, a caliper within the range of from about 3 to 10 mils and a density within the range of from about 0.5 to 0.8 gram per cubic centimeter) is about 45 to 50 percent based on the weight of resin solids in a completely dry resin impregnated sheet. Sheets, however, having a much greater range of percentage of resin solids in the sheet, for example sheets having resin contents of from 35 to 65 percent may be used. The base sheet may be impregnated with the resin in any conventional manner, for example by passing the paper through a bath of an aqueous solution of the resin and then through the nips of one or more roll pairs to adjust the resin content of the impregnated paper. While the paper is normally first manufactured and then impregnated with the resin in accordance with the method above described or similar methods, the resin may alternatively be incorporated into the beater of the papermaking machine and the resin impregnated paper made in this manner.

In the case of paper impregnated by contacting the paper with a liquid solution or emulsion of the resin, following the impregnation, the paper may be passed through a drying operation. The purpose of the drying operation is to reduce the volatiles in the impregnated paper to the point where the paper can be readily handled, shipped and stored without the inconvenience that would accompany this handling of damp paper. In additlon to these considerations, the volatile content of the sheet must be reduced to the point where moisture from the resin impregnated attachment sheet will not blister the surfacing sheet during the attachment of the surfacing lam nate to the particle board. On the other hand, the drying operation can not be so drastic as to result in a substantial curing of the resin, in which case the resin would not serve the purpose of adhesively attaching the surfacing sheet to the particle board in the final operatron. It is therefore desirable that the impregnated attachment sheet be dried to a volatile content of less than about 15 percent. It is also essential that this dryin operatlon not reduce the flow of the resin in the imoreg nated paper to less than 2 percent. The basis weight of the impregnated sheet should be between about 50 and 150 pounds per 3000 square feet and sheets in the range of to pounds per 3000 square feet are preferable. As previously pointed out in connection with the base paper a single sheet of resin impregnated paper having this basis weight can readily be prepared and is preferred. However, if desired, a plurality of lighter weight sheets can be laminated together to form an impregnated attachment sheet within this basis Weight range.

The surfacing sheet should have certain characteristics. The surface must be a smooth one, that is free from fuzz, imperfections, etc., if the material is to accept paint readily and be smooth after painting. The density of the paper should be quite high in order to prevent strikethrough from the resin of the attachment sheet. The tensile strength of the sheet should be good and there should be a high resistance to internal rupture in order to prevent delamination of the sheet. The maximum bonding between the sheet and paint applied to the surface is also attained if the sheet has substantially little or no resin on the outer surface. Finally, the sheet should be rela tively inexpensive and accordingly have a low basis weight. It has been found that grease-proof paper sheets in a basis weight of between about 20 and 40 pounds per 3000 square feet and preferably between about 25 and 30 pounds per 3000 square feet and having a thickness between about 1.2 and 2.5 mils and preferably about 1.5 mils has these desired characteristics. The term greaseproof paper is used in the present specification and claims to indicate a paper made from chemical wood pulps which are highly hydrated in order that the resulting paper may be resistant to oil and grease. The term is not intended to include other papers of low hydration characteristics which are treated with resins or with other materials, either by impregnation or by coating, to render these papers made from low hydrated pulps more resistant to organic materials. Pulp stocks which are employed to make the greaseproof paper of the present invention are beaten, refined or otherwise hydrated for long periods of time of the order of 4 to 18 hours. The length of the beating period depends upon the pulp, the type of refining equipment and the end use specification of the paper. The hydration characteristics may be determined by the porosity of the paper produced from the pulp. This porosity is measured according to the Technical Association of Pulp and Paper Industry Standard 479 sm-48 using the Gurley-Hill S-P-S permeability tester and should measure in excess of 3600 seconds using 20 ounce cylinder and a 1 square inch orifice. The greaseproofness of the paper may be determined by the TAPPI Standard turpentine test T454 m-44. Suitable papers as measured by this test should have values between about 8 and 45 minutes and the values preferably between about 20 and 30 minutes. The greaseproof paper may be either bleached or unbleached and may be opaque in order to improve its masking characteristics. The surfacing sheet of greaseproof paper should have a density between about 0.90 and 1.10 grams per cubic centimeter as determined by TAPPI method T411 m 44.

The surfacing laminate of the present invention is especially designed to be applied to particle board, particularly that particle board having a density within the range of about 35-55 pounds per cubic feet. Its utility is not so limited, however, since the laminate may be applied to other lignocellulosic board such as hard-board, panel board, building board, etc., where it is desired to mask the surface of the board and to provide a smooth paintable surface for the lignocellulosic board. Where the laminate of the present invention is applied to a low density board it may be desirable to modify the method of attachment of the laminate to the board since the temperatures and pressures normally employed with particle board may permanently compress the low density board. Thus a resin impregnant which is activated at a lower pressure and temperature or a separate glue line might be employed to attach the laminate to low density board.

The lignocellulosic board is surfaced in accordance with the process of the present invention by placing the attachment sheet on the surface of the board and the surfacing sheet overlaying the attachment sheet and then subjecting the assembly or lay-up" as it is usually known,

to heat and pressure sufificient to uniformly bond the assembly into a composite whole. This may be accomplished by inserting the assembly in a conventional plywood press and pressing the assembly for a desired time at the requisite pressure and temperature. Caul plates may be employed, and are desirable if the surface plates of the press are not smooth. While both faces of the particle board are normally surfaced in one operation, one face may be surfaced if desired. The purpose of the pressing operation is to cause suflicient flow of the resin impregnant of the attachment sheet so that the resin migrating to the surfaces of the attachment sheets uniformly contacts both the surfacing sheet and the particle board over the entire area of the attachment sheet and the resin is then cured to its irreversible stage so that the board and surfacing laminate becomes a composite, unified whole. When the resin impregnated attachment sheet contains one of the usual laminating resins such as the phenol-aldehyde type, or the resorcinol-aldehyde type, consolidation of the board and laminate may be accomplished at a temperature of approximately 275- 325 F., under a pressure of -175 pounds per square inch in a time about 3 to 10 minutes. It is to be understood, of course, that practical equivalent time-temperature relationships may be employed. Should other laminating resins be employed the curing temperature, pressure, and time may be adjusted to the particular resin involved.

Some additional convenience may be obtained if the resin impregnated attachment sheet and greaseproof surfacing sheet are adhesively bonded together into a surfacing laminate, prior to the bonding of the laminate to the particle board base. This may be accomplished by bringing the greaseproof surfacing sheet and resin impregnated attachment sheet into contact and partially curing the resin in the attachment sheet under heat and pressure. It is essential, however, that the resin not be carried to the fully cured state if the laminate is to be attached to the particle board by means of resin flow from the attachment sheet. It is of course possible to attach the resin impregnated attachment sheet to the surfacing sheet and the laminate to the particle board by means of a glue line applied to either of the engaging surfaces.

Now that the present invention has been generally described it may be further illustrated by an example.

Example In the practice of the invention on a commercial scale the surfaced particle board product was made by assembling on a caul a layer of greaseproof paper surfac- 1ng sheet and superposed upon this layer a resin impregnated attachment sheet, a sheet of /2 inch particle board, a resin impregnated attachment sheet and a greaseproof paper surfacing sheet and a second caul. This assembly was placed in a conventional plywood press and pressed for 5 minutes at a temperature of C., under a pressure of 150 pounds per square inch. The particle board was a /2 inch particle board made of wood shavings and bits and bonded together with phenol formaldehyde resin in accordance with conventional commercial practice. Both sheets of resin impregnated paper employed as the attachment sheets were identical and were of 90 pounds basis weight per 3000 square feet on an impregnated basis. The base sheets Were made from a pulp furnish of 45 percent sulfite, 40 percent kraft and 15 percent groundwood to a basis weight of 43 pounds per 3000 square feet. The sheets were then impregnated with a phenol formaldehyde laminating resin to a resin content of 48 percent resin solids in the resin impregnated sheet on a volatile free basis. The resin employed was a conventional phenol formaldehyde resin, Resinox 468 manufactured by the Monsanto Chemical Corporation. The resin impregnated sheets were air dried to a volatile content of 15 percent. Between the time of the impregnation and the time of lamination the volatile content had decreased to about 10 percent. The greaseproof surfacing sheets employed had a basis Weight of 25 pounds per 3000 square feet and a thickness of 1.5 mils and a density of 1.0 g./cc. The greaseproof paper was made from a bleached sulfite fiber finish. The greaseproofness of the paper determined in accordance to the TAPPI Standard test T454 m-44 was 20 minutes and the Gurley-Hill porosity as measured by TAPPI Standard T479 sm-48 was in excess of 3600 seconds. The resulting surface particle board was characterized by having a smooth uniform easily paintable surface.

In order to obtain a surface having increased wet strength over that of conventional greaseproof paper a small amount of wet strength resin may be added to the greaseproof surfacing sheet in accordance with known practices in the papermakin-g art. Some of these wet strength resins are of the beater addition type although others are added to the sheet by impregnating operation. Insofar as production of the masking sheets of the present invention are concerned the presence or absence of these wet strength producing additives and the percentages normally employed to 3 weight percent of the dry weight of the cellulose) has little effect. Also, it makes no difference whether the additive is of a resinous or other nature.

The surfacing laminate described in the foregoing description makes possible the attainment of a very smooth, uniform surface on particle board and other similar materials. dark colors; it may include decorative effects; and it does provide a readily paintalble surface not only with respect to the adherence of the paint or enamel to the surface but also with respect to the smoothness of the resulting surface. Furthermore, the surfacing laminate -is of sufficient opacity that it readily masks all imperfections of the particle board surface and can be manufactured and applied to the particle board or other lignocellulosic sheet materials with which they are used at relatively low cost commensurate with the cost of the particle board.

What is claimed is:

1. A composite lignocellulosic fibrous board comprising a particle board core, at least one greaseproof paper surfacing sheet consisting .of highly hydrated wood pulp attached to said core, and means for adhesively attaching said sheet to at least one side of said core, said attaching means consisting of at least one layer of paper impregnated with a thermoset laminating resin and interposed between said core and said sheet, the outer surface of said sheet being substantially free of resin.

2. A particle board panel having a surface which is smooth and uniform which comprises a particle board base and a surfacing laminate integrally attached to at least one surface of said particle board, said laminate The surface provided may be either light or consisting of at least one resin impregnated paper attachment sheet containing about 35-65 percent of a thermoset laminating resin, and a grease-proof paper surfacing sheet consisting of highly hydrated wood pulp and having a basis weight of about -40 pounds per 3000 square feet and a density of 0.90-1.10 grams per cubic centimeter, the outer surface of said surfacing sheet being substantially free of resin.

3. A particle board panel having a smooth paintable surface, which comprises a particle board base and a surfacing laminate integrally attached to at least one surface of said particle board, said laminate comprising a resin impregnated paper attachment sheet having a basis weight of about 50-150 pounds per 3000 square feet, a thickness of from about 3 to 10 mils, and containing about -65 percent of a thermoset aldehyde laminating resin, and a dense, smooth surfacing sheet consisting of grease-proof paper of highly hydrated pulp, said surfacing sheet havin-g a basis weight of about 20-40 pounds per 3000 square feet, a thickness of about 1.2-2.5 mils and characterized by a porosity in excess of 3600 seconds as measured by TAPPI Standard T479 sm-48 and a grease-proofness as measured by TAPPI Standards T454 m-44 of about 8-45 minutes, the outer surface of said surfacing sheet being substantially free of resin.

4. The particle board panel of claim 3 in which the resin is a phenol-aldehyde resin.

5. The particle board panel of claim 3 in which the resin is a resorcinol-aldehyde resin.

6. A particle board panel having a smooth paintable surface which comprises a particle board core, and a surfacing laminate integrally attached to the exposed surfaces of said particle board, said surfacing laminate consisting of an attachment sheet and a grease-proof surfacing sheet, said attachment sheet having a basis weight of about 80-100 pounds per 3000 square feet, a thickness of about 5-8 mils, and being impregnated with a phenol-aldehyde resin in an amount of about 45-50 percent of resin solids per dry sheet, said grease-proof paper surfacing sheet consisting of highly hydrated pulp and being substantially free of resin on its outer surface, said surfacing sheet having a basis weight of about 25-30 pounds per 3000 square feet, a thickness of about 1.5 mils, and a Gurley-Hill porosity in excess of 3600 seconds.

References Cited in the file of this patent UNITED STATES PATENTS 1,851,177 Harvey et al. Mar. 29, 1932 2,456,006 Hickler Dec. 14, 1948 2,559,109 Bonini July 3, 1951 2,565,251 Malmstrom Aug. 21, 1951 2,656,296 Grangaard Oct. 20, 1953 2,745,779 Ritter et al. May 15, 1956 2,774,698 Jenk et al. Dec. 18, 1956 

1. A COMPOSITE LIGNOCELLULOSIC FIBROUS BOARD COMPRISING A PARTICLE BOARD CORE, AT LEAST ONE GREASEPROOF PAPER SURFACING SHEET CONSISTING OF HIGHLY HYDRATED WOOD PULP ATTACHED TO SAID CORE, AND MEANS FOR ADHESIVELY ATTACHING SAID SHEET TO AT LEAST ONE SIDE OF SAID CORE, SAID ATTACHING MEANS CONSISTING OF AT LEAST ONE LAYER OF PAPER IMPREGNATED WITH A THERMOSET LAMINATING RESIN AND INTERPOSED BETWEEN SAID CORE AND SAID SHEET, THE OUTER SURFACE OF SAID SHEET BEING SUBSTANTIALLY FREE OF RESIN. 